1. Field of the Invention
The present invention relates to the display technical field, and in particular to a pixel driving circuit and a pixel driving method of an organic light-emitting diode.
2. The Related Arts
As a new generation of display technology, active matrix/organic light-emitting diode, AMOLED, has advantages of High brightness, wide color gamut, a wide viewing angle, fast response, small size, etc. the light emitting device of AMOLED is organic light-emitting diode, OLED, under AMOLED driving circuit driving, when the current flows through the OLED, the OLED lights up. AMOLED is a current driven device, the brightness is decided by the current size flowing through the OLED, most of the existing integrated circuit, IC, only transmit the voltage signal; therefore, AMOLED pixel circuit usually adopts 2T1C V/I conversion circuit to transfer the voltage signal to the current signal.
Please refer to FIG. 1 which is a traditional 2T1C V/I conversion circuit. It comprises a switching transistor (T1′), a driving transistor (T2′), a storage capacitor (C1′) and an organic light emitting diode (OLED′); it also comprises a scanning control terminal (Scan′), a data signal terminal (Data′), a power supply voltage (VDD′) and a power supply negative electrode (VSS′); the switching transistor (T1′) comprises a first gate (g1′), a first source (s1′) and a first drain (d1′), the driving transistor (T2′) comprises a second gate (g2′), a second source (s2′) and a second drain (d2′); the first gate (g1′) is electrically connected with the scanning control terminal (Scan′), the first source (s1′) is electrically connected with the data signal terminal (Data′), the first drain (d1′) is electrically connected with the second gate (g2′) and the lower plate of the storage capacitor (C1′); the power supply voltage (VDD′) is electrically connected with the upper plate of the storage capacitor (C1′) and the second source (s2′); the anode of the organic light emitting diode (OLED′) is electrically connected with the second drain (d2′), the cathode of the organic light emitting diode (OLED′) is electrically connected with the power supply negative electrode (VSS′). The driving transistor (T2′) is used to make sure the driving current of AMOLED panel driving circuit, the organic light emitting diode (OLED′) is used to response the driving current to light up and display; the storage capacitor (C1′) mainly stores the gray scale voltage signal output by the data signal terminal (Data′), the driving current size of the driving transistor (T2′) is decided by the gray scale voltage stored in the storage capacitor (C1′); the switching transistor (T1′) and the driving transistor (T2′) are thin film transistors, TFT.
Please refer to FIG. 2 and combine FIG. 1, which is the traditional 2T1C V/I conversion circuit simulation data sheet, wherein Vth′ is the threshold voltage, IOLED′ is the current flowing through the organic light emitting diode (OLED′), ΔIOLED′ is the current drift rate flowing through the organic light emitting diode (OLED′): ΔIOLED2′=(IOLED2′−IOLED1′)/IOLED1′, ΔIOLED3′=(IOLED3′−IOLED1′)/IOLED1′. According to the simulated date in FIG. 2 shows that the threshold voltage (Vth) of the driving transistor drifts, the current variation is too large, the circuit fails.
Therefore, this simply designed pixel is sensitive to the threshold voltage (Vth) of the thin film transistor, the channel mobility, the startup voltage of the OLED, the quantum efficiency and the transient of the power supply. Since the threshold voltage of the driving transistor (T2′) will drift with the working time, thus resulting the emitting instability of the organic light emitting diode (OLED′); otherwise, since the existence of the leakage current of the switching transistor (T1′), it makes the voltage of the storage capacitor (C1′) unstable, thus resulting the emitting instability of the organic light emitting diode (OLED′), too. Moreover, since the drift of the threshold voltage of the driving transistor (T2′) of each pixel is different, increasing or decreasing, it makes the luminous between each pixel is uneven. Therefore, using such 2T1C pixel circuit without compensation, the unevenness of the AMOLED brightness is about 50% or greater.
One way to solve the unevenness is to add the compensation circuit to each pixel, the compensation means that it must compensate the driving thin film transistor parameters (such as threshold voltage and mobility) of each of pixels, making the output current is independent of this parameters.